Determination of Small Amounts of Carbon-14 in the Presence of Large Iodine-131 Activities, and an Experimental Estimation of the Specific Activities of (¹³¹I)-Iodotyrosines

Abstract

IN the examination of mechanisms of chemical reactions, and particularly in biochemical studies of this nature, radioactive tracer substances of high specific activity are needed. In many cases it is desirable to use doubly labelled tracers containing carbon-14 or tritium as well as iodine-131. Recently, in an investigation of thyroid hormones (¹³¹I)-iodotyrosines of very high specific activity were prepared by Reith and Tampion¹. As the specific activity of these compounds could only be calculated from uncertain values, it was necessary to attempt an experimental determination using ¹⁴C-tyrosine in the iodination procedure. However, even using a carbon-14 substance of the highest obtainable specific activity, the iodine-131 activity is so much higher that their simultaneous counting becomes an extremely difficult if not an impossible task. Taking the specific activity of carbon-14 tyrosine as 20 mc./m.mole, an average iodination would give a relation of 0.01 µc. carbon-14 for 500 µc. iodine-131; that is, a ratio of activities of approximately 1 : 50,000 on liquid scintillation counting. So it was necessary to find a method of eliminating as much of the iodine-131 as possible prior to scintillation counting of the carbon-14. The following method allows the elimination of more than 99.9 per cent of the counts of iodine-131, making it possible to count even small amounts of ¹⁴CO₂ in a multichannel liquid scintillation spectrometer. The method can easily be used in serial determinations of several dozen experiments per day.